A light emitting element using a light emitting material has advantages of thinness, lightness in weight, fast response, direct-current low-voltage driving, and so on, and is expected to be applied to a next-generation flat panel display. Further, a light emitting device having light emitting elements arranged in matrix is superior to a conventional liquid crystal display device in a wide viewing angle and high visibility.
A light emitting element has the following light-emission mechanism: voltage is applied to a light emitting layer sandwiched between a pair of electrodes, so that electrons injected from a cathode and holes injected from an anode are recombined in a light-emission center of the light emitting layer to form molecular excitons; thus, light is emitted by releasing energy when the molecular exciton returns to the ground state. As the excited state, a singlet-excited state and a triplet-excited state are known, and the light emission is possible in either of the excited states.
Emission wavelength of a light emitting element is determined by energy difference between a ground state and an excited state, that is, a band gap, of a light emitting molecule included in the light emitting element. Therefore, various emission colors can be obtained by devising structures of the light emitting molecules. By manufacturing a light emitting device using light emitting elements each capable of emitting red light, blue light, and green light, which are the three primary colors of light, a full-color light emitting device can be manufactured.
However, there is a problem in a full-color light emitting device that a light emitting element with excellent color purity can not always be manufactured easily. This is because it is difficult to realize a light emitting element with high reliability and excellent color purity, although light emitting elements for red, blue and green with excellent color purity are needed for manufacturing a light emitting device having superior color reproducibility. As a result of recent development of materials, light emitting elements for red and green with high reliability and excellent color purity have achieved with respect to light emitting elements for red and green. However, as to a light emitting element for blue with high reliability and excellent color purity has not been realized, so many studies have been done (For example, Patent Document 1: Japanese Patent Laid-Open No. 2003-31371).